1. Field of the Invention
The present invention relates to a wafer spinner for fabricating a semiconductor device, and more particularly, to a wafer spinner which coats the entire surface of a wafer with a nearly uniform layer of photoresist by controlling the temperature of the wafer.
2. Discussion of the Related Art
Photolithography is used to form a precise predetermined pattern on a specific material layer as part of a semiconductor device fabricating process. The pattern must be precise because the device has very minute structures. Photolithography processes use photochemical reactions of a photoresist to establish the pattern on the layer and to remove the photoresist after the pattern has been formed. Photolithography apparatus include a stepper for performing photolithography and a spinner for coating a photoresist on a wafer.
In the general photolithography process, after the photoresist is coated on the wafer, a pattern is projected onto the photoresist using light, thus exposing a portion of the photoresist. During the development process, either the exposed or unexposed portion of the photoresist is removed, depending on the specific application. Thereafter, a specific material is deposited and the remaining photoresist is removed, leaving a patterned layer.
More specifically, the photolithography process starts by coating a wafer with a photoresist. The coating operation is usually performed using a wafer spinner to rotate the wafer and spread the photoresist along the surface of the wafer. The wafer is oriented horizontally on the spinner's rotation plate comprising a vacuum chuck for holding the wafer in place by vacuum action. The spinner is then rotated and a predetermined amount of photoresist is supplied from above the center of the wafer. The photoresist spreads over the entire surface of the wafer due to the centrifugal force of the rotating wafer.
It is important for the spinner to coat the wafer with a predetermined, uniform thickness of photoresist over the entire surface of the wafer. If the photoresist is too thin, some of the unexposed region may be removed while removing the exposed portion during the development process. If the photoresist is too thick, a portion of the pattern is not exposed sufficiently, leaving a photoresist pattern wider than the predetermined pattern after the development process. This produces a pattern wider or narrower than a designed line width, or produces a pattern with a missing element. As a result, the semiconductor device may malfunction. Moreover, if the photoresist is not coated on the wafer uniformly, it is difficult to set the standards for the following processes. Even if the standards are set optimally, the greater deviation in the structure widths will lead to more chips on the wafer that deviate farther from the standards, which results in greater numbers of defects and reduced yields.
FIG. 1 is a schematic cross-sectional view of a conventional spinner. A rotation-plate vacuum chuck 13 is connected to a driving motor 11 through a rotation shaft 12 at the center of the spinner. A wafer 10 is loaded on the rotation chuck 13. Only a portion of the wafer 10 near the center of the wafer 10 is placed on the rotation chuck 13. A bowl (upper cup) 14 is placed over the wafer, and a lower cup 15 is placed under the wafer. The cups 14 and 15 prevent cleaning water or photoresist from splashing out of the apparatus during processing.
An inner cup 16 is placed under the rotation chuck 13 and surrounds it, to prevent the photoresist from damaging the driving motor 11 placed under the rotation chuck. The surplus cleaning water and photoresist are guided by the inner cup 16, bowl 14 and lower cup 15, to drain through a drain outlet 17 placed below the lower cup 15.
FIG. 2 is a cross-sectional view showing a wafer 20 on which a photoresist 21 is coated using the conventional spinner. The photoresist supplied to the center of wafer 20 is a sol-state (colloidal) substance containing a volatile solvent. The photoresist spreads on the wafer due to the centrifugal force generated by rotating the wafer with the spinner, and the centrifugal force drives the liquid to the edge of the wafer. Since the photoresist has surface tension, some photoresist may accumulate on the edge of the wafer. After a period of time, the volatile solvent evaporates and a semi-solid photoresist layer 21 is formed on wafer 20. FIG. 2 shows that the photoresist layer 21 near the edge of the wafer is thicker than in other areas. The distance from the center A of the wafer 20 to the location B on the wafer 20 where the semi-solid photoresist begins to thicken due to surface tension is called the flat-zone radius. The area of the wafer within the flat-zone radius is the flat zone.
Because of the thicker semi-solid photoresist layer near the edge, it is difficult to produce uniform chips in both the flat zone and the outer wafer edge beyond the flat zone radius B of the same wafer 20. This variation leads to an increase in the probability of creating poor quality chips. This problem becomes more serious as the semiconductor devices become more highly integrated, and as the diameter of the wafer increases.
FIG. 3 is a cross-sectional view showing a wafer 30 on which a photoresist layer 31 is coated using the conventional spinner while its rotation shaft is heated by the motor. As in the case above, the sol-state photoresist is supplied at the center A of the wafer. The fluidity or viscosity of the photoresist layer 31 depends inversely on the temperature. In other words, the fluidity of the liquid photoresist decreases with higher temperature, and the thickness of the layer 31 coated increases, because the solvent contained in the liquid photoresist evaporates more quickly at the higher temperature. For example, for a 1000 .ANG. thick layer of photoresist that is deposited, about 100 .ANG. of additional semi-solid photoresist is created and coated on the wafer 30 for each 1.degree. C. of temperature difference.
As wafers 30 are sequentially loaded onto the spinner and the spinner continues to operate to coat photoresist on the loaded wafers 30, the driving motor 11 (in FIG. 1) continues to generate heat that causes its temperature to rise. This heat is transmitted to the rotation chuck 13 and wafers 10 through the rotation shaft 12. The transmitted heat increases the temperature of the liquid photoresist deposited near the center A of the wafer 30, compared to the photoresist that spreads to the outer portions of the wafer 30, which in turn increases the thickness of the photoresist 31 in the center A. Here, the temperature is increased from a range of about 21 to 22.degree. C. (ambient temperature) during the initial operation of the spinner, to a range of about 25 to 26.degree. C. later in the process. Thus, the thickness variation of the photoresist layer 31 from center-to-edge is about 400 .ANG. as a result of the 4.degree. C. increase. This variation exceeds the designed quality control standard of .+-.50 .ANG. for the photoresist layer, and frequently causes poor quality chips.
FIG. 4 is a cross-sectional view of a conventional spinner having a motor flange 41 for cooling its driving motor 11. Cooling water flows in the direction of the arrows through the motor flange 41 located on driving motor 11 to thereby remove the heat generated in the driving motor 11. Though a portion of heat is transmitted to the rotation chuck 13 through the rotation shaft 12, the temperature increase is mitigated somewhat by the cooling water. Accordingly, referring to FIG. 3, it is possible to prevent the thickness of the photoresist layer 3 coated near the center A of wafer 30 from increasing as much. However, even in this case, the photoresist layer at the outer regions of the wafer is thicker than the portion of the wafer between the center and edge of the wafer. Again, such a non-uniform photoresist layer results in greater numbers of wafer defects and reduced yields.